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arXiv · 2607.25841

General Relativistic Entropic Acceleration at the perturbation level: a CLASS implementation and first Boltzmann-code constraints

Simone D'Onofrio·Dong Ha Lee·Eleonora Di Valentino·Juan García-Bellido
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Abstract

General Relativistic Entropic Acceleration (GREA) attributes the late-time acceleration of the Universe to the entropy growth of the causal cosmological horizon, without a cosmological constant, with a phenomenology fixed by the single $\mathcal{O}(1)$ parameter $\alpha$. The model has so far been confronted with data only at the background level. We present its first implementation within an Einstein-Boltzmann solver: the GREA background is integrated directly into CLASS, while the entropic component is evolved as an effective fluid regulated by the parametrized-post-Friedmann scheme, giving access to the full CMB and matter power spectra. A Markov-chain Monte Carlo analysis with COBAYA against the full primary-CMB likelihoods, DESI DR2 BAO and Type Ia supernovae constrains the coupling $\alpha \sim 1$, in excellent agreement with the theoretical prediction, with a fit matching $\Lambda$CDM to within $|\Delta\chi^2| \lesssim 6$ despite the addition of a single free parameter. The equation of state inferred from the data agrees with binned, model-independent reconstructions and exhibits a second crossing of the phantom divide at $z \simeq 2$, a distinctive prediction of the thermodynamic dynamics rather than of an imposed parametrization.

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Simone D'Onofrio, Dong Ha Lee, Eleonora Di Valentino, Juan García-Bellido. 2026-07-28. General Relativistic Entropic Acceleration at the perturbation level: a CLASS implementation and first Boltzmann-code constraints. https://arxiv.org/abs/2607.25841

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